I've seen some on amazing before age 6-12 months. If you've got time then have a good look online at different shops, I'm sure you'll manage to find some. Also the waistband on pants is very stretchy so you may get away with a slightly bigger size x
Size 18 should be good, a little bigger will make it easier for him to learn how to pull them down
https://www.bambinomio.co.uk/products/the-give-it-a-go-bundle